cache: add tracing to prune #5898

Job	Run time
build (windows/amd64)	1m 45s
build (freebsd/amd64)	1m 42s
sandbox-build (linux/amd64)	6m 57s
sandbox-build (linux/arm64)	6m 44s
test-freebsd-amd64	2m 42s
test-windows-amd64 (containerd, ./client)	7m 22s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 8s
test-windows-amd64 (containerd, ./worker/containerd)	2m 46s
test-windows-amd64 (containerd, ./solver)	2m 19s
test-windows-amd64 (containerd, ./frontend)	2m 15s
test-windows-amd64 (containerd, ./frontend/dockerfile)	40m 6s
test-windows-amd64 (./..., 1)	4m 40s
	1h 22m 26s

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